How to Loop a Specified Number of Times with for in C# - dummies

How to Loop a Specified Number of Times with for in C#

By John Paul Mueller, Bill Sempf, Chuck Sphar

The while loop is the simplest and second most commonly used looping structure in C#. Compared to the for loop, however, the while loop is used about as often as metric tools in an American machine shop.

The for loop has this structure:

for(initExpression; condition; incrementExpression)

{

// . . . body of code . . .

}

When the for loop is encountered, the program first executes the initExpression expression and then executes the condition. If the condition expression is true, the program executes the body of the loop, which is surrounded by the braces immediately following the for command. When the program reaches the closed brace, control passes to incrementExpression and then back to condition, where the next pass through the loop begins. In fact, the definition of a for loop can be converted into this while loop:

initExpression;

while(condition)

{

// . . . body of code . . .

incrementExpression;

}

An example in C#

You can better see how the for loop works in this example:

// Here is one C# expression or another.

a = 1;

// Now loop for awhile.

for(int year = 1; year < duration; year = year + 1)

{

// . . . body of code . . .

}

// The program continues here.

a = 2;

Assume that the program has just executed the a = 1; expression. Next, the program declares the variable year and initializes it to 1. Then the program compares year to duration. If year is less than duration, the body of code within the braces is executed. After encountering the closed brace, the program jumps back to the top and executes the year = year + 1 clause before sliding back over to the year < duration comparison.

The year variable is undefined outside the scope of the for loop. The loop’s scope includes the loop’s heading as well as its body.

Why do you need another loop?

Why do you need the for loop if C# has an equivalent while loop? The short answer is that you don’t — the for loop doesn’t bring anything to the table that the while loop can’t already do.

However, the sections of the for loop exist for convenience — and to clearly establish the three parts that every loop should have: the setup, exit criteria, and increment. Not only is this arrangement easier to read, it’s also easier to get right. (Remember that the most common mistakes in a while loop are forgetting to increment the counting variable and failing to provide the proper exit criteria.)

The most important reason to understand the for loop is that it’s the loop everyone uses — and it (along with its cousin, foreach) is the one you see 90 percent of the time when you’re reading other people’s code.

The for loop is designed so that the first expression initializes a counting variable and the last section increments it; however, the C# language doesn’t enforce any such rule. You can do anything you want in these two sections; however, you would be ill advised to do anything but initialize and increment the counting variable.

The increment operator is particularly popular when writing for loops. The previous for loop is usually written this way:

for(int year = 1; year < nDuration; year++)

{

// . . . body of code . . .

}

You almost always see the postincrement operator used in a for loop instead of the preincrement operator, although the effect in this case is the same. There’s no reason other than habit and the fact that it looks cooler. (The next time you want to break the ice, just haul out your C# listing full of postincrement operators to show how cool you are. It almost never works, but it’s worth a try.)

The for loop has one variation that you may find hard to understand. If the logical condition expression is missing, it’s assumed to be true. Thus for(;;) is an infinite loop. You see for(;;) used as an infinite loop more often than while(true).